During engine operation, acoustic pressure oscillations at undesirable frequencies can develop in the combustion chamber due to, for example, burning rate fluctuations inside the combustion chamber. Such pressure oscillations can damage components of the combustion chamber. To avoid such damage, one or more acoustic damping devices can be associated with the combustion chamber of a gas turbine. One type of commonly used acoustic damping device is a Helmholtz resonator. Various examples of Helmholtz resonators are disclosed in U.S. Pat. Nos. 6,530,221 and 7,080,514.
A Helmholtz resonator for use in a combustion chamber of a gas turbine engine is disclosed in EP 1862739 B1, wherein the resonator comprises a resonator cavity and a resonator neck. The resonator neck has a circular cross section with a plurality of circumferentially spaced cooling holes formed in the tube wall. In use, the resonator is thus continually purged with cooling air passing through the array of holes. The purging air keeps the resonator cavity at a temperature at which no thermal damage occurs and beneficially creates a flow of air in the neck that travels from the cavity to the combustion chamber both cooling the neck and preventing ingestion of hot combustor gasses.
A Combustion device for a gas turbine is disclosed in EP 2295864 B1, which comprises a portion provided with a first and a second wall provided with first passages connecting the zone between the first and second wall to the inner of the combustion device and second passages connecting said zone between the first and second wall to the outer of the combustion device, wherein between the first and second wall a plurality of chambers are defined, each connected with one first passage and at least one second passage, and defining a Helmholtz damper, wherein the chambers are defined by at least a first plate interposed between the first and second wall, and the chambers are defined by holes indented in said first plate.
EP 0 892 216 discloses a combustion device with a first and a second wall with first passages connecting the zone between the walls to the inside of the combustion device and second passages connecting the zone between the walls to the outside of the combustion device. EP 0 892 216 also discloses plates between the first and second walls to define chambers (honeycomb structure) that are connected to the first and second passages.
In fact, during operation of the combustion chamber, hot gases may enter from the combustion chamber into the neck between the resonator cavity and the combustion chamber and go out again, coming back into the combustion chamber, which is referred to as hot gas ingestion (HGI). Usually when this occurs, the hot gases may change the temperature inside the resonator cavity, by which the resonance frequency of the Helmholtz resonator is changed. Thus, the damping performance of the damper is decreased. Furthermore, hot gas ingestion may cause severe thermal fatigues to the component of the resonator that usually made of regular materials.
Hot gas ingestion is conventionally prevented by using sufficiently large cooling air to purge the neck in order to suppress the ingestion of the hot gas. However, the volume of the cooling air cannot be chosen to be much large due to deterioration of the damping performance and emission increase of NOx caused thereby.
Even much development has been made in this field, there still exists a need for a damper for combustion oscillation damping that may restrain hot gas ingestion and that further may be made in a cost efficient way.